Printed circuit boards are today manufactured by a process which is almost completely automated. The circuits for a particular printed circuit board are generated using a computer aided design (CAD) machine which will not only generate a schematic of the printed circuit board but also will provide the board layout for all of the devices thereon. The printed circuit board layout information is provided to a device such as a laser plotter which exposes the artwork needed to fabricate the printed circuit board. The artwork comprises a series of transparent and opaque areas as features corresponding to the PCB devices.
However, defects may be present in the printed circuit board or the PCB artwork which would render the printed circuit board useless. These defects can have a variety of causes, including shrinkage in the artwork or failures in the fabrication process. Many known systems for defect detection in printed circuit boards have often simply compared a given printed circuit board against a reference, defect free printed circuit board (i.e. golden board) to detect errors created during the fabrication process. A Golden Board System does not detect errors in subsequent boards that were also in the golden board.
Moreover, some of the known optical PCB inspection systems are configured to examine inspection marks which are placed on the artwork away from the features. The dimensions and locations of the registration marks on the printed circuit board are compared against a reference to determine the extent of shrinkage. Should the shrinkage exceed a certain value, the board is considered by these systems to be defective.
In order to create an image of the PCB features, the CAD data must be translated into raster format and provided to a laser direct imager (LDI) such as the LDI 9720 or 9725 marketed by the Gerber Scientific Instrument Company, the assignee of the present invention. The comparison of devices on the printed circuit board and features on the reference image is performed by a device such as the model 1850 defect detection system, also marketed by the Gerber Scientific Instrument Company.
A technique for generating a three state transitional data base (TDB) is disclosed and claimed in the commonly owned U.S. Pat. No. 5,157,762 entitled "A Method and Apparatus for Providing a Three State Database for use with Automatic Optical Inspection Systems" and is incorporated herein by reference. That technique is characterized by an algorithm which creates from a two state database a three state (black, white, gray) TDB with each feature having a single, uniform tolerance.
Another method and apparatus used with printed circuit board (PCB) defect detection is disclosed and claimed in U.S. Pat. No. 5,163,128 and is incorporated herein by reference. The '128 system generates a reference database image of the PCB having tolerances for each individual PCB feature and is characterized by a transitional data base having three states, black, white and gray corresponding to areas where the PCB features respectively must appear, must not appear and may or may not appear.
These systems are limited in the fact that certain defects will not be found. For example, if adjacent features are separated by a distance less than the sum of the corresponding feature tolerances, a short therebetween will not be detected by known three state transition database systems since the short is coextensive with the feature tolerances.
It would be advantageous to have a method and apparatus for creating a database from the original raster CAD data for use in defect detection of printed circuit boards which would provide the capability of generating a weighted transition database allowing for more precise detection of defects. The present invention is directed towards such a method and apparatus.